In the field of the optical pickup apparatus, a wavelength of a laser light source used as a light source for reproducing information recorded on an optical disc, and for recording of information on an optical disc, has been shortened in recent years, and laser light sources each having a wavelength of 400-420 nm such as, for example, a violet semiconductor laser and a blue SHG laser wherein a wavelength of an infrared semiconductor laser is converted by the use of a second harmonic are being put to practical use. If these violet laser light sources are used, information of 15-20 GB can be recorded on an optical disc with a diameter of 12 cm, when an objective lens having numerical aperture (NA) identical to that of DVD (digital versatile disc) is used, and information of 23-25 GB can be recorded on an optical disc with a diameter of 12 cm, when NA of the objective lens is raised up to 0.85. Hereafter, an optical disc and a magneto-optical disc both using a violet laser light source are generically called “high density optical disc”, in the present specifications.
Incidentally, in the case of the high density optical disc-using an objective lens with NA 0.85, an amount of comatic aberration generated by a skew of an optical disc is increased, and therefore, there are some optical discs wherein a protective substrate is designed to be thinner than that in DVD (0.1 mm for the thickness 0.6 mm in DVD) and an amount of comatic aberration caused by skew is reduced. Meanwhile, capability of only conducting recording and reproducing of information properly for the high density optical disc of this kind is not sufficient in terms of value as a product of an optical disc player and recorder. In view of the fact that DVD and CD (compact disc) in which various pieces of information are recorded are on the market presently, capability of only conducting recording and reproducing of information for the high density optical disc is not sufficient. For example, capability of conducting recording and reproducing of information properly even for DVD and CD owned by a user leads to enhancement of a commercial value as an optical disc player and recorder for the high density optical disc. From this background, it is desired that an optical pickup apparatus mounted on an optical disc player and recorder for a high density optical disc has capability for conducting recording and reproducing of information properly while maintaining compatibility for the high density optical disc and DVD, and further, compatibility for the high density optical disc and CD.
As a method to conduct recording and reproducing of information properly while maintaining compatibility between a high density optical disc and DVD, and further compatibility between a high density optical disc and CD, there is considered a method to switch selectively between an optical system for a high density optical disc and an optical system for DVD or CD depending on recording density of an optical disc for which recording and reproducing of information is conducted. However, this method is disadvantageous for downsizing of a device and for cost reduction, because a plurality of optical systems are needed.
Therefore, for the purpose of simplification of the structure of the optical pickup apparatus and of cost reduction, it is preferable to reduce the number of optical parts constituting the optical pickup apparatus to the utmost, by making an optical system for a high density optical disc and optical systems for DVD and CD to be common, even in the case of the optical pickup apparatus having compatibility. What is most advantageous for simplification of the structure of the optical pickup apparatus and for cost reduction is to make objective lenses each being arranged to face an optical disc to be common and to make this objective lens to be of the single lens structure. Incidentally, to obtain an objective lens which is compatible for optical discs of plural types each being different from others in terms of a wavelength for recording and reproducing, it is necessary to form a phase structure having wavelength dependency of spherical aberration.
Now, Patent Document 1 discloses an objective lens that is the single lens structure and is capable of conducting recording and/or reproducing of information for a high density optical disc, DVD and CD on a compatible basis.
(Patent Document 1) TOKKAI No. 2004-79146
In the technology disclosed by the Patent Document 1, diffraction efficiency is enhanced for a light flux emitted from an infrared laser light source for blue and DVD (hereinafter referred to as “red”) and for a light flux emitted from an infrared laser light source for CD (hereinafter referred to as “infrared” by utilizing even-order-numbered diffracted light for a light flux emitted from a violet laser light source for a high density optical disc (hereinafter referred to as “blue”). However, a diffraction angle that is determined by “diffraction order number×wavelength” becomes substantially the same for blue and infrared because a wavelength ratio of blue to infrared is about 2:1, and compatibility between a high density disc and CD is difficult to be achieved in diffraction operations. Therefore, in the objective lens in the prior art, compatibly for both a high density optical disc and CD is achieved by causing “infrared” to enter as divergent light. However, this method for compatibility has a problem that an amount of generation of comatic aberration is large when the objective lens is subjected to tracking driving in the case of using CD, and it is not practical.